The present invention relates to quality assurance instruments for medical radiotherapy equipment used for radiation treatment of tumors or the like.
Cancerous tumors may be treated by irradiating the tumor with high-energy photons or electrons (henceforth both termed “radiation”).
Such radiotherapy relies in part on the fact that tumor tissue is more sensitive than normal tissue to such high-energy radiation. Nevertheless, the radiation dose must be carefully controlled to limit the exposure of healthy tissue while ensuring sufficient radiation is received by the tumor.
Radiation dose may be controlled by a variety of means including shutters for collimating the radiation beam to the area of the tumor, filters for varying the intensity of radiation within the area of the tumor, and control of the exposure duration. An accurate understanding of the energy, flux, and alignment of the radiation beam is essential for such control. Generally, as is understood in the art, radiation energy describes the average energy of the individual photons or electrons whereas radiation flux is number of electrons or photons per unit area per unit time.
Radiation energy may be determined by calculating changes in flux at two depths within a homogenous medium, for example, a water phantom.
Radiation flux is normally determined using an ionization chamber or semiconductor detector placed in the radiation beam at a fixed distance from the radiation source. A “build-up” material such as a plastic block may be placed in front of the flux-detector to improve its sensitivity. For the purposes of periodic quality assurance of a radiotherapy machine, the output of the flux-detector may be compared to a base line for the same detector. In this way, precise calibration of the detector to a standard is not required.
Radiation alignment is normally determined with respect to a visible light field projected along with the radiation showing, for example, an illuminated rectangular area and/or cross-hair pattern. Alignment may be verified by exposing a film marked to show the location of the light field or crosshairs and comparing the exposed film to the markings. Alternatively, as shown in U.S. Pat. No. 4,988,866, a fixture having multiple ionization detectors and multiple light detectors (also called edge detectors) may be used, and the signals from the ionization detectors and light detectors may be compared.
It is desirable that the radiation therapy machine be checked on a frequent, periodic basis at each of its settings. Such quality assurance checks can be cumbersome and time consuming particularly when multiple pieces of test equipment must be used, for example, as would be required to calibrate a radiotherapy machine that provides both electron beams and photon beams at a variety of energy levels. It is difficult to construct a quality assurance instrument that works for a wide variety of different radiation energies and different radiation modes, e.g. electrons or photons, equally well.